The invention relates to a soldering method, and a soldering method for repairing a component of a gas turbine damaged both by high thermal as well as mechanical stresses.
Turbine blades of gas turbines are subject to high thermal and mechanical loads during operation. Because of different extreme stresses, these loads result in various material defects. On the one hand, high fluctuations in temperature and centrifugal forces result in tension that causes tears. In addition, oxidation, corrosion, erosion or abrasion may cause larger material defects, such as recesses, holes, or other disadvantageous types of material removal. To extend the life span of the damaged parts and prevent a cost-intensive replacement with new parts, the state of the art provides many repair methods that strive to eliminate material defects and also restore the original shape of the turbine blade.
U.S. Pat. No. 4,726,101 discloses a method for repairing a turbine blade which utilizes a tape consisting of soldering powder and a binder applied to the turbine components and is heated. The base material and the tape fuse by way of diffusion during the heat treatment that follows. In order to lower the fusion point of the soldering powder, which principally has a similar composition as the base material, 1.5% boron is added to it. Use of several layers of tape is also possible. The tape contains 30-40% plastic binder (organic binder) that must evaporate during the subsequent soldering process and which may leave behind undesirable pores or residues (carbon, ash), resulting in reduced strength and quality. The heat treatment also causes the tape to shrink.
Also known are several sintering processes that use soldering powders. Such a method is described, for example, in publication U.S. Pat. No. 5,156,321. The soldering powder is applied to the surface of the base material and is then heated to such an extent that the fusion temperature of the soldering powder is exceeded, i.e., it melts, while the base material remains solid. The liquid soldering materials sinters together and by way of a capillary effect in the tear, diffuses into an area of the base material near the latter""s surface. This creates a relatively dense solder layer. Also known from publication U.S. Pat. No. 4,381,944 are other methods for repairing tears using two alloys that are present in powder form. However, joints made with soldering powder are less advantageous in that they are not sufficiently temperature-resistant, i.e., the ability of the soldering joint to manage stress deteriorates as the temperature increases.
A method which utilizes with a tape or, optionally, pre-sintered soldering powder, is also known from publication U.S. Pat. No. 5,666,643. For additional stability, particles with a high fusion point are added to the soldering material. Although pre-sintered soldering powders do not typically shrink, they have the disadvantage that they are inflexible when cold, i.e., it is difficult to adapt them to a predefined contour such as the contour presented by a component such as a turbine blade.
Many alloys are known that are principally suited for soldering processes. For example, U.S. Pat. No. 3,155,491, discloses such a soldering alloy.
The present invention provides a soldering method and material for repairing components, such as damaged turbine blades, that can be used in a simple manner and avoids the disadvantages associated with the state of the art.
According to one aspect, the present invention provides a soldering method for repairing tears and material defects in a surface of thermally and mechanically highly stressed components comprising a base material, the method comprising the steps of:
(a) applying an open-celled metallic foam to the tear or material defect;
(b) introducing of soldering powder into the pores of the metallic foam; and
(c) heating the powder containing foam to a temperature which is above the melting temperature of the soldering powder.
According to another aspect, the present invention provides in combination: a highly thermally and mechanically stressed component formed of a base material; and
an open-celled metallic foam applied to a surface of the base material.
According to the present invention, it is advantageous for the metallic foam to be shapeable and easily adaptable to the contour of the base material. In one advantageous embodiment, the open-pored metallic foam has a porosity of 60-95%. This ensures a sufficient capillary effect of the metallic foam.
According to a further aspect, a filler is added to the soldering powder and is introduced into the pores of the metallic foam. In another advantageous embodiment, the composition was selected so that the filling material and/or the metallic foam are metallurgically close to the composition of the base material of the component to be repaired.
According to the present invention, it may be advantageous to apply small amounts of a binder to the surface of the metallic foam after the soldering powder has been filled in. This is particularly effective in preventing the unintentional falling out of soldering powder during careless handling. The applied binder is able to simply evaporate, since it has only been applied to the surface, and therefore need not diffuse out of the pores of the metallic foam.
By adapting the chemical composition of the metallic foam, the method according to the invention also can be used advantageously to specially protect areas subject to extreme oxidation.